1. Field of the Invention
This invention relates generally to the manufacture of ceramic items and particularly to improvements in the manufacture of thin flat ceramic substrates by the tape casting process.
2. Description of the Prior Art
The art of manufacturing thin flat ceramic items has been developed to meet the need for capacitor dielectrics and for substrates in microelectronic and semiconductor circuitry, among other uses. The common method for manufacturing such articles is by the tape casting process. In this process a fluid ceramic slurry, called a slip, is made by mixing together a finely ground ceramic powder, such as aluminum oxide (alumina), with suitable amounts of an organic binder, a volatile solvent, usually a plasticizer, and possibly small amounts of other materials, depending on the product requirements. Typical ingredients are identified, and the mixing and casting process are fully described in U.S. Pat. No. 2,966,719, issued to J. L. Park, Jr. on Jan. 3, 1961 as assignor to American Lava Corporation, and in U.S. Pat. No. 3,698,923, issued to H. W. Stetson et al. on Oct. 17, 1972 as assignors to Western Electric Company, Incorporated. The reader is referred to these patents and also to the earlier U.S. Pat. No. 2,582,993, issued to G. N. Howatt on Jan. 22, 1952, for details of the tape casting process and its developmental history as well as for the characteristics of the resulting ceramic product.
Briefly, the process involves discharging the abovedescribed ceramic slip, which has a viscosity and consistency approximately the same as heavy cream, from a reservoir onto a supported, moving surface, preferably a plastic tape or film such as cellulose acetate, polytetrafluoroethylene ("Teflon"), or glycol terephthalic acid ester ("Mylar"). The film is usually in the form of an elongated strip several hundred feet long and from 1/2 to 2 feet wide, wound on a storage reel mounted next to the reservoir.
The tape is led from the storage reel under the reservoir to a takeup reel, and a suitable drive mechanism moves the tape in a substantially horizontal path from the storage reel to the takeup reel. The cast slip is distributed evenly on the moving tape by an inverted dam forming the outlet of the reservoir or by a doctor blade in order to form a layer of uniform and controlled thickness.
As the layer of cast slip is conveyed on the plastic tape from the reservoir, the volatile solvents evaporate, the process of driving off the solvents being accelerated by passing the tape through an elongated, heated drying chamber. The resulting product is a ceramic tape that is aptly described as "leather hard". This tape can be punched or sliced into the shape and size desired for the substrate or other item and then be fired at high temperature (e.g. 1500.degree.C) to produce a rigid ceramic article.
The critical factor limiting the production speed of dried, "green" ceramic tape (i.e. leather-hard tape) is the rate of evaporation of the solvents from the layer of cast slip. In conventional drying chambers the traditional means for heating the slip are warm flowing air or infra-red heaters. Both of these methods aggravate the drying problem by heating and drying the surface before the bulk of the layer has been warmed to the evaporation temperature of the solvent. This surface heating further dries and hardens the skin that tends to form even in ambient air temperatures and thereby retards evaporation from the still fluid interior of the layer.
As a result, in order to obtain reasonable tape speeds, typically of about 2 feet per minute, through the drying chamber, the tape casting production line must be as much as 150 feet long to provide the necessary time within the drying chamber for all the volatile solvents to evaporate. If hotter air is used to bring the temperature of the slip up more quickly, the surface drying and hardening effect is more pronounced, and more rapid evolution of solvent vapors from the interior tends to produce bubbles in the skin and voids in the interior of the tape product.